This project investigates the neural mechanisms that subserve the monkey's ability to discriminate noxious thermal stimulation. In the present investigation, single neurons were identified in the primary somatosensory cortex (SI) of the anesthetized monkey. Neurons with slowly adapting responses to intense mechanical stimunation received an intraderamal injection of capsaicin into the receptive field. Capsaicin is known to only active c-fibers and therefore is a purely nonciceptive stimulus. The receptive fields of nociceptive neurons were classified as either wide dynamic range (WDR), preferentially responsive to only noxious stimuli. Nociceptive neurons that responded to intradermal capsaicin injections were additionally tested for responses to noxious thermal stimulation. A very small percentage of neurons responsive to capsaicin injection responded to noxious thermal stimulation. In past experiments a response to noxious thermal stimulation was the criterion for a neuron to be classified as nociceptive. These date suggest that nociceptive nurons are common in the somtosensory cortex. These data contrast with the long held view that nociceptive neurons are rare in the primary somatosensory cortex. Nociceptive S1 neurons were somatotopically organized. NS neurons were only found in the anterior half of the distribution of WDR neurons. The majority of neurons were located in the middle layers (III & IV) and tended to be found in clusters. The differences in response properties and the locations of WDR and NS neurons suggest that they may subserve different transient changes in noxious thermal stimulation. In addition, NS neurons may be involved in the process of localization and cortical activation that accompany noxious thermal stimulation.